In general, in the manufacturing processes of a semiconductor device, a fine pattern is formed using photolithography. For the formation of a fine pattern, many substrates called as photomasks are normally used. Herein, a substrate used for formation of a fine pattern using photolithography, such as a photomask, and the substrate provided with a transfer pattern is referred to as “a transfer mask”. In general, this transfer mask is equipped with a fine pattern made of a metal thin film or the like on a transparent glass substrate. Photolithography is used for the manufacture of a transfer mask.
For the manufacture of a transfer mask by photolithography, a mask blank is used. The mask blank has a thin film, such as light shielding film, for formation of a transfer pattern on a transparent substrate, such as a glass substrate. Herein, a transfer pattern may also be referred to as “a mask pattern”. Manufacture of a transfer mask using the mask blank comprises steps of: exposing to form a desired pattern to a resist film formed on a mask blank; developing the resist film in accordance with the desired pattern to form a resist pattern; etching the thin film in accordance with the resist pattern; and stripping and removing the remained resist pattern. In the developing step, after forming the desired pattern to the resist film formed on the mask blank, developer is supplied to dissolve a portion of the resist film soluble in the developer for formation of the resist pattern. In the etching step, using this resist pattern as a mask, a portion of the exposed thin film where the resist pattern is not formed is dissolved by dry etching or wet etching, thereby forming the desired mask pattern on the transparent substrate. Thus, a transfer mask is produced.
In the steps of manufacturing a glass substrate for a mask blank used for manufacture of the transfer mask, cleaning is performed for the glass substrate after mirror polishing for removing particles, such as a foreign substance, for example, on a surface of the substrate. Conventionally, several methods are known for the cleaning.
For example, in Japanese Laid-Open Patent [Kokai] Publication No. 2000-173965, a cleaning method using a high speed shear flow is disclosed. In the cleaning method using a high speed shear flow described in Japanese Laid-Open Patent [Kokai] Publication No. 2000-173965, objects to be cleaned and high pressure nozzles are disposed at predetermined intervals in a process tank mainly of ultrapure water, and high speed shear flow of the ultrapure water injected from the high pressure nozzles are generated near the surfaces of the objects to be cleaned. With the high speed shear flow, fine foreign substances attached to the surfaces of the objects to be cleaned are stripped by breaking the bond with the surfaces of the objects to be cleaned. Further, the high speed shear flow prevents the removed foreign substances from reattaching to the surfaces of the objects to be cleaned.
In Japanese Laid-Open Patent [Kokai] Publication No. 2007-201186, an apparatus for cleaning a substrate is disclosed, which cleans a substrate by supplying a cleaning fluid to one surface of a surface to be cleaned. The apparatus for cleaning substrate of Japanese Laid-Open Patent [Kokai] Publication No. 2007-201186 comprises a cleaning fluid supply unit supplying a cleaning fluid to the surface of the substrate to be cleaned, equipped towards a surface of a substrate to be cleaned, and a cleaning roller equipped towards the surface of the substrate to be cleaned so as to be approximately parallel to the cleaning fluid supply unit and also feeding the cleaning fluid into a gap formed with the surface of the substrate to be cleaned by rotation.
In addition, as a cleaning method for removing particles on a substrate surface, there is a method of cleaning a substrate surface by striking cleaning water with ultrasonic of about 1 MHz applied directly to the substrate surface. This cleaning is referred to as “megasonic cleaning”.
As one example of the megasonic cleaning, Japanese Laid-Open Patent [Kokai] Publication No. 2001-96241 discloses a method of cleaning a precision substrate, such as a mask blank substrate, using a cleaning fluid made by mixing ozonized water or anode water with hydrogenated water or cathode water, and applying ultrasonic to the cleaning fluid. In addition, Japanese Laid-Open Patent [Kokai] Publication No. 2005-221928 discloses that the megasonic cleaning is performed, for example, with supplying a cleaning fluid, with ultrasonic applied thereto, to a surface of a transparent substrate, after etching process on a surface of a transparent substrate using an etching fluid, as an example of physical cleaning utilizing a physical action for removing foreign substances attached on the substrate surface.
In addition, in recent years, as a method of cleaning a substrate to clean a substrate surface, such as a semiconductor wafer, a two-fluid-jet cleaning method is used. For example, a cleaning method for removing contaminants on a substrate surface using the two-fluid-jet cleaning method is described in Japanese Laid-Open Patent [Kokai] Publication No. 2008-226900.
On the other hand, in recent years, because of a demand for miniaturization of semiconductor devices, there is an exposure technique using an extreme ultraviolet light. Hereinafter, an extreme ultraviolet is referred to as “EUV”. EUV lithography is highly expected, which is one of reflective lithography. Here, EUV light refers to light in a wavelength band of the soft x-ray region or the vacuum ultraviolet region, and specifically to light having a wavelength approximately from 0.2 to 100 nm. As a mask used for the EUV lithography, a reflective mask for exposure described in Japanese Laid-Open Patent [Kokai] Publication No. 2002-122981, for example, is proposed.
The reflective mask described above has a multilayer reflective film to reflect exposure light formed on a substrate and has an absorber film with a pattern formed on the multilayer reflective film to absorb exposure light. Light introduced to a reflective mask mounted to an exposure apparatus (a pattern transfer apparatus) is absorbed in an area with the absorber film, and an optical image reflected by the multilayer reflective film in an area without the absorber film through a reflective optical system is transferred on a semiconductor substrate.